1. Field of the Invention
The present invention relates to a process for preparing trehalulose and isomaltulose. More specifically, the invention relates to the process for preparing a high trehalulose syrup which comprises converting sucrose into trehalulose and isomaltulose using a sugar-conversion enzyme produced by novel microorganisms.
2. Description of the Prior Art
Isomaltulose and trehalulose are found in honey, are taste sweet and are chemically classified as hetero-disaccharides, which are composed of glucose and fructose. These components are linked with an .alpha.-1,6-glucosidic linkage in isomaltulose, and with an .alpha.-1,1-glucosidic linkage in trehalulose.
It has been well known that isomaltulose is odontologically non-cariogenic. Recently trehalulose was also demonstrated to have non-caries inducing properties in in vitro and in vivo experiments. These facts promoted a wide use of trehalulose and isomaltulose-containing syrup called "Isomaltulose Syrup".
Trehalulose and isomaltulose are usually formed simultaneously from sucrose through glucosidic transfer catalyzed by .alpha.-glucosyltransferase found in some specific microorganisms, for example Protaminobacter rubrum, Serratia spp. and Erwinia spp. German Patent No. 1049800, published in 1959, firstly described a microbiological process for producing crystalline isomaltulose from sucrose using some specific microorganisms represented by Protaminobacter rubrum. Japanese Patent Kokoku No. 58-38156 discloses a continuous aerobic culture process for converting sucrose to isomaltulose using a bacterial strain selected from a group comprising Protaminobacter, Serratia, Erwinia and Leuconostoc. Japanese Patent Kokoku No. 60-9797 also describes a production method of isomaltulose using immobilized enzyme derived from a strain of Erwinia spp. Japanese Patent Kokoku No. 57-10720 discloses a process for preparing isomaltulose which comprises culturing a microorganism belonging to the genus Protaminobacter or Serratia in a sucrose solution under aerobical condition. In these conventional processes, the syrup is produced as a subsidiary product.
In these processes particularly suited to crystalline isomaltulose production, a ratio of trehalulose to isomaltulose in quantity formed from sucrose by enzymic conversion lays in the range of 1:6 to 1:10. After most of isomaltulose is crystallized and centrifuged off from the reaction mixture, the residual matter is obtained in a form of molasses containing trehalulose in a concentration substantially higher than isomaltulose.
Remaining syrup in which crystalline isomaltulose obtained by this conventional method has been recovered contains isomaltulose usually in a concentration near saturation and then isomaltulose is apt to crystallize during its storage, because of having lower solubility of isomaltulose in water. Such phenomena often may cause a lowered commercial value of the product and a difficulty of pumping transportation. In order to avoid such disadvantage, it is necessary to prepare a trehalulose syrup having a concentration of isomaltulose below the saturation point.
Furthermore, all crude isomaltulose solutions obtained in enzymic treatments of sucrose in these conventional processes usually contain considerable amounts of glucose, fructose and isomaltose in addition to larger components of isomaltulose and trehalulose. The presence of glucose and fructose, which represent about 5% of the total saccharide content, causes a considerable reduction of the production efficiency, and moreover shows a heavy color formation during processes operated at higher temperatures for recovering isomaltulose in crystalline forms from the solution. Therefore large capacity decolorization units are required in a conventional isomaltulose production factory, in particular wherein the residual matter is processed into a syrup for use in foodstuff.
Generally the residual matter obtained finally from the process of isomaltulose recovery can be processed into a food grade syrup in a series of procedures composed of decolorization, deionization, filtration and evaporation. The syrup contains all components found originally in the crude isomaltulose solution, while the main component of the syrup is not isomaltulose, but trehalulose.
It is well known that isomaltulose can be used suitably in a variety of foods, particularly in low-cariogenic foods as a sole substitute of sucrose in their conventional formulations.
However it is undesirable that the solubility of isomaltulose in water is considerably lower than that of sucrose. Thus crystalline preparations of isomaltulose has been seldom used in such foods as jams, marmalades, gelatins, youkan (a gelatin-like Japanese sweet food) and other foods which traditionally contain sucrose in a high concentration, because the deteriorative recrystallization of isomaltulose is expected to occur in these foods in storage, when they are prepared by using isomaltulose wholly or partly in place of sucrose in their conventional formulations.
Trehalulose and isomaltulose preparations, in particular high trehalulose syrups, if abundantly available, are considered to be more readily substituted for sucrose in such foods as mentioned above than crystalline isomaltulose, because of the high solubility in water and the preferable sweet taste of trehalulose. Trehalulose rich preparations, as high trehalulose syrups, are therefore required increasingly by food manufacturers, while they are still provided only in limited quantities in the conventional processes.